This program is concerned with the neuronal circuitry in spinal cord and thalamus of cats and monkeys that subserves somatic sensation. Synaptic populations in normal brain are first characterized and quantified by light and electron microscopic techniques. Neurons and synapses of a particular population are then labeled by special stains, by transport of radioactive proteins or certain enzymes within axons, or by degeneration of neurons, axons and synapses following specific lesions. In this manner, the way in which neurons are interconnected can be determined, in order to understand the neuronal mechanisms subserving pain and other somatic sensations.